It is known to provide a carburettor in which air from an inlet is arranged to pass through both a main duct and an auxiliary duct into an inlet manifold for an internal combustion engine. Fuel is introduced into the auxiliary duct through a tube and is mixed with the air flowing therethrough, the resulting fuel/air mixture being mixed with the air from the main duct in the inlet manifold, and the mixture then being valved into the cylinders of the engine. A carburettor of this kind is described in our U.S. Pat. No. 3,721,428 in which a main butterfly valve is provided in each of the ducts, a further butterfly valve is provided in the inlet, and an auxiliary butterfly valve is provided in the auxiliary duct upstream of the fuel injection tube. We have found that the auxiliary butterfly valve can be used for controlling the fuel/air mixture, and in our aforementioned specification we described an arrangement for manually setting the auxiliary butterfly valve to control the mixture at idling engine speeds, and for automatically changing the setting of the valve with changing engine speed. However, in view of the present stringent requirements for fuel economy and pollution control, we have devised an improved carburettor in which the fuel/air mixture for the engine is controlled automatically to maintain optimum performance of the carburettor.
We have found the following operating parameters of an internal combustion engine each provide an indication of the richness of the carburetted mixture required to be introduced into the engine: